2.6 Addressing


IP addresses are 32-bit (in the case of IPv4) and 128-bit (in the case of IPv6) unsigned binary values. For example, the following shows an IPv4 and an IPv6 address: 199.174.41.5 and 12AB:0:0:CD30:123:4567:89AB:CDEF. IP addressing for IPv4 is described in RFC 116 and for IPv6 in RFC 2373. IP addresses are essentially hierarchical in nature, which provides scalability to the addressing architecture. IP addresses are made up of two parts , a network part and a host part. The network part of the IP address identifies the network to which the host is connected; all hosts attached to a network have the same network ID in their address. The host part identifies each host uniquely on that network. Let's look at addressing in IPv4 and IPv6 networks separately.

2.6.1 IPv4 Addressing Architecture

There are two types of addresses from a uniqueness perspective: globally routable addresses and private IP addresses. Globally routable addresses are unique on a global scale. Private addresses are one of a set of solutions for the diminishing address space in IPv4 networks. These addresses are not globally unique and are usually used by enterprises and small networks that do not directly connect to the Internet. Three ranges have been set up for this: 10.0.0.0, 172.16.0.0 to 172.16.31.0, and 192.168.0.0 to 192.168.255.0.

The hierarchical nature of the address architecture created a class structure, A, B, C, and D as described next . The class of the IP address is identified by the most significant bits (MSBs) in its address.

  • If the first bit is 0, then it is a class A address. It has a has a 7-bit network number and a 24-bit local address, thus allowing 128 class A networks to be created.

  • If the first bit is 1 and the second is 0, then it is a class B address. It has a 14-bit network number and a 16-bit local address. This allows 16,384 class B networks.

  • Is the first two bits are 1 and the third is 0, it is a class C address. It has a 21-bit network number and an 8-bit local address. This allows 2,097,152 class C networks.

  • The fourth type of address, class D, is used as a multicast address. The four highest-order bits are set to 1-1-1-0.

2.6.2 Subnetting

Due to the rapid growth of the Internet and as a result of the fast depletion of IP addresses, the principle of assigned IP addresses became inflexible . To avoid having to obtain more IP addresses for networks, subnetting was introduced. Subnetting is a mechanism in which the address structure is subdivided into a second network number and a host number. With subnetting the address now appears as

Network number, subnetwork number, host number

The subnetwork division is visible only to the internal network, and from a routing perspective the network number is still the key to routing packets to a destination. Subnetting is accomplished using a 32-bit subnet mask. Bits with a value of zero bits in the subnet mask indicate positions ascribed to the host number. Bits with a value of 1 indicate positions ascribed to the subnet number. The bit positions in the subnet mask belonging to the original network number are set to ones but are not used. Like IP addresses, subnet masks are usually written in dotted decimal form. Two types of subnetting are in common use: static subnetting and variable-length subnetting.

  • In static subnetting the subnets obtained from the same network use the same subnet mask. This is less optimal since it tends to waste address space in small networks.

  • Variable-length subnetting allows subnets within the same network to use different subnet masks. It divides the network such that each subnet contains sufficient addresses for the required number of hosts.

2.6.3 Classless Internet Domain Routing (CIDR)

The rapid growth in terms of the number of hosts and networks in the 1990s made designers rethink the hierarchical class-based address architecture of IPv4. Unless some form of aggregation was enabled, there was a serious possibility of running out of addresses by the mid-1990s. Standard routing was based on class A, B, and C network addresses. Subnetting provides a greater degree of granularity within these networks. However, it is not possible to specify that multiple class C addresses are related . This results in an explosion in the routing table entries in the backbone. If a class B network requires 1 entry in the backbone, an equivalent network of class C address ranges would require 16 entries.

CIDR helps in aggregating routes. It enables a single entry in the forwarding table to reach a set of different networks. It achieves this by getting rid of the class structure of the IPv4 addresses. It is based solely on the high-order bits of the IP address. These bits are termed the IP prefix. CIDR is specified in RFCs 1518, 1519, and 1520. The implementation of CIDR in the Internet is primarily based on the Border Gateway Protocol Version 4 (BGP-4). The mechanism of combining multiple networks via the use of network masks is sometimes refered to as supernetting. With the introduction of CIDR, IPv4 addresses are no longer classified as being of type A, B, or C.

2.6.4 IPv6 Addressing Architecture

IPv6 addresses are 128 bits as compared to the 32-bit IPv4 addresses. This significant increase in address size is expected to be sufficient for a long time. With 128 bits, it is possible to have 340 undecillion addresses as compared to the 4 billion IPv4 addresses. The IPv6 addressing architecture is specified in RFC 2373.

Three different types of addresses can be assigned to an IPv6 interface (note that addresses are assigned to interfaces and not nodes) :

  • Unicast address

  • Multicast address

  • Anycast address

UNICAST ADDRESSES

IPv6 unicast addresses are aggregatable with contiguous bitwise masks similar to IPv4 addresses under classless interdomain routing. There are several forms of unicast address assignment in IPv6, including the global aggregatable unicast address, the Network Service Access Point (NSAP) address, the Internet Packet Exchange (IPX) hierarchical address, the site-local address, the link-local address, and the IPv4-capable host address. The global unicast address can be aggregated; the format of the aggregation is as shown in Figure 2-8.

Figure 2-8. Aggregation in IPv6.

graphics/02fig08.gif

In Figure 2-8,

001 : Format prefix (3 bit) for aggregatable global unicast addresses

TLA ID : Top-level aggregation identifier

RES : Reserved for future use

NLA ID : Next-level aggregation identifier

SLA ID : Site-level aggregation identifier

INTERFACE ID : Interface identifier

ANYCAST ADDRESSES

IPv6 introduces the concept of an anycast address. This is not found in IPv4. An IPv6 anycast address is an address that is assigned to more than one interface (typically belonging to different nodes), with the property that a packet sent to an anycast address is routed to the " nearest " interface having that address, according to the routing protocols' measure of distance. Anycast addresses are allocated from the unicast address space, using any of the defined unicast address formats. Thus, anycast addresses are syntactically indistinguishable from unicast addresses.

MULTICAST ADDRESSES

IPv6 gets rid of the concept of broadcast addresses since broadcast is a variant of multicast anyway. An IPv6 multicast address is an identifier for a group of nodes. A node may belong to any number of multicast groups. Multicast addresses have the format shown in Figure 2-9.

Figure 2-9. IPv6 multicast address.

graphics/02fig09.gif

The first 8 bits set to 1 indicate a multicast address. Flags are a set of four flags. The scope is used to limit the scope of the multicast group, and the group ID identifies the multicast group.



IP in Wireless Networks
IP in Wireless Networks
ISBN: 0130666483
EAN: 2147483647
Year: 2003
Pages: 164

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